用MOCVD在r面蓝宝石衬底上生长的a面GaN的独特电学性质研究

Nonpolar （1120） a-plane GaN films have been grown by low-pressure metal-organic vapor deposition on r-plane （1102） sapphire substrate. The structural and electrical properties of the a-plane GaN films are investigated by high-resolution X-ray diffraction （HRXRD）, atomic force microscopy （AFM） and van der Pauw Hall measurement. It is found that the Hall voltage shows more anisotropy than that of the c-plane samples; furthermore, the mobility changes with the degree of the van der Pauw square diagonal to the c direction, which shows significant electrical anisotropy. Further research indicates that electron mobility is strongly influenced by edge dislocations.     

MOCVD生长的[ 1-102]r面蓝宝石衬底上的[11-20 ]a面GaN表面形貌研究

用原子力显微镜和高分辨XRD分析了生长在r面蓝宝石上的a面GaN,a面是无极性的,和c面的最大不同就是无极化电荷,以及由于极化引起的导带弯曲。分别采用高温AlN和低温GaN作为缓冲层生长a面GaN, 用高温AlN作为缓冲层的a面GaN出现了三角坑的表面形貌,而用低温GaN作为缓冲层则出现了褶皱的形貌。对不同缓冲层a面GaN的缺陷形成原因进行了讨论,并且确定了三角坑和褶皱条纹的晶向。     

High-Quality GaN heteroepitaxial films grown by metalorganic chemical vapor deposition

In this paper, we describe the growth and characterization of high-quality GaN heteroepitaxial films grown on basal-plane sapphire substrates using metalorganic chemical vapor deposition. The quality of these films is analyzed by a variety of methods, including high-resolution x-ray diffraction, optical transmission spectroscopy, transmission electron microscopy (TEM), room temperature photoluminescence, and room-temperature Hall measurements. The x-ray diffraction full width at half maximum value of ΔΘ ～37 arc s is the narrowest reported to date for any III-V nitride film on any substrate. The x-ray rocking curves for ～0.48 μm thick GaN/Al₂O₃ heteroepitaxial layers exhibit Pendellösung fringes, indicating that even relatively thin films can be of high quality. High-resolution TEM lattice images further attest to the excellent structural quality, showing the films to be completely free of stacking faults. Furthermore, no evidence of columnar growth is observed.     

Epitaxial growth of nonpolar GaN films on r-plane sapphire substrates by pulsed laser deposition

Single-crystalline nonpolar GaN epitaxial films have been successfully grown on r-plane sapphire (Al₂O₃) substrates by pulsed laser deposition (PLD) with an in-plane epitaxial relationship of GaN[1-100]//Al₂O₃[11-20]. The properties of the ~500 nm-thick nonpolar GaN epitaxial films grown at temperatures ranging from 450 to 880 °C are studied in detail. It is revealed that the surface morphology, the crystalline quality, and the interfacial property of as-grown ~500 nm-thick nonpolar GaN epitaxial films are firstly improved and then decreased with the growth temperature changing from 450 to 880 °C. It shows an optimized result at the growth temperature of 850 °C, and the ~500 nm-thick nonpolar GaN epitaxial films grown at 850 °C show very smooth surface with a root-mean-square surface roughness of 5.5 nm and the best crystalline quality with the full-width at half-maximum values of X-ray rocking curves for GaN(11-20) and GaN(10-11) of 0.8° and 0.9°, respectively. Additionally, there is a 1.7 nm-thick interfacial layer existing between GaN epitaxial films and r-plane sapphire substrates. This work offers an effective approach for achieving single-crystalline nonpolar GaN epitaxial films for the fabrication of nonpolar GaN-based devices.     

Edge-emitting electroluminescence polarization investigation of InGaN/GaN light-emitting diodes grown by metal-organic chemical vapor deposition on sapphire (0001)

The edge-emitting electroluminescence (FL) state of polarization of blue and green InGaN/GaN light-emitting diodes (LEDs) grown in EMCORE’s commercial reactors was studied and compared to theoretical evaluations. Blue (∼475 nm) LEDs exhibit strong EL polarization, up to a 3:1 distinction ratio. Green (∼530 nm) LEDs exhibit smaller ratios of about 1.5:1. Theoretical evaluations for similar InGaN/GaN superlattices predicted a 3:1 ratio between light polarized perpendicular (E⊥c) and light polarized parallel (E‖c) to the c axis. For the blue LEDs, a quantum well-like behavior is suggested because the E⊥c mode dominates the E‖c mode 3:1. In contrast, for the green LEDs, a mixed quantum well (QW)-quantum dot (QD) behavior is proposed, as the ratio of E⊥c to E‖c modes drops to 1.5:1. The EL polarization fringes were also observed, and their occurrence may be attributed to a symmetric waveguide-like behavior of the InGaN/GaN LED structure. A large 40%/50% drop in the surface root mean square (RMS) from atomic force microscopy (AFM) scans on blue/green LEDs with and without EL fringes points out that better surfaces were achieved for the samples exhibiting fringing. At the same time, a 25%/10% increase in the blue/green LED photoluminescence (PL) intensity signal was found for samples displaying EL interference fringes, indicating superior material quality and improved LED structures.     

Local vibrational modes in Gamma-irradiated GaN grown by metal-organic chemical vapor deposition

We have studied the effects of gamma-ray irradiation on Fourier transform infrared grazing incidence reflection–absorption (FTIR-GIRA) spectra of the GaN films grown on α-Al₂O₃ (0 0 0 1) substrates using the atmospheric pressure metal-organic chemical vapor deposition process. After gamma-ray irradiation, the intensities of FTIR-GIRA spectra are enhanced and a series of new absorption features appeared. Among them, those at 3440, 3355, 3088, 2819, 2712, 2100, 1426 and 894 cm⁻¹ are discussed and identified. The reason why gamma-ray irradiation can enhance the intensities of and cause new features in FTIR-GIRA spectra of GaN films is discussed.     

Characteristics of GaN stripes grown by selective-area metalorganic chemical vapor deposition

We report on the selective-area metalorganic chemical vapor deposition of GaN stripes in the size range of 50 to 125 urn and the characterization of the morphology, topography, and optical properties of these stripes. GaN films (∼1–3 μm) grown on (0001) sapphire are used as the substrates. Excellent surface morphology is achieved under optimized growth conditions which include a higher V/III ratio than broad area growth. It is found that, under certain growth conditions, (0001) terraces of ~5 μm in width develop at the edges of all stripes, independent of stripe size and orientation. The selectively grown GaN yields stronger band-edge emission than the “substrate” GaN which indicates an improvement in optical quality. However, the donor-acceptor pair recombination (or conduction band to acceptor transition) and yellow emission are also enhanced in certain areas of the stripes. The spatial correlation of these emission bands is established by cathodoluminescence wavelength imaging, and the origin of these emissions is speculated.     

GaN薄膜制备及脉冲激光沉积法的研究进展

介绍了国内外制备GaN材料的历史和现状。分析了GaN材料及其薄膜的各种制备工艺的特点,并详细介绍了采用激光沉积工艺制备GaN薄膜的研究进展。     

Mg注入非极性a面GaN退火温度的研究

非极性GaN材料解决了传统GaN材料中的极化现象,具有较好的应用前景。用金属有机物化学气相沉积方法,在r面蓝宝石上生长了Mg注入非极性a面GaN 薄膜,并选取650 ℃、750 ℃、850 ℃这3个温度对Mg注入GaN薄膜进行退火温度研究。用原子力显微镜、光致发光谱、拉曼谱研究了材料的表面形貌、光学性质以及面内应力。结果表明,在750 ℃退火时效果较好。     

Observation of compensation in GaN films grown by metalorganic chemical vapor deposition

Hall-effect measurements were conducted on an unintentionally doped n-type GaN films grown on (0 0 0 1) sapphire substrates in a low-pressure metalorganic chemical vapor deposition (LP-MOCVD) system. Variable temperature Hall-effect measurements reveal a different dependence of the electron concentration and the Hall mobility on temperature for the two samples. In the sample with a background concentration of 10¹⁸ cm⁻³, the electron concentration shows a non-monotonous relationship with increasing temperature from 90 to 400 K. However, in the normal sample with a background concentration of <10¹⁷ cm⁻³, the dependence of the electron concentration on temperature is monotonous between 90 and 400 K. The different behavior was also found in the dependence of the Hall mobility on temperature for the sample. The experimental data were analyzed by considering two donor levels and one acceptor level model. The good agreement between measured data and calculation demonstrates that the abnormal dependence is indicative of a compensation effect in GaN film grown by LP-MOCVD.     

热壁化学汽相沉积Si基GaN奇异面

利用热壁化学汽相沉积在Si基上生长GaN薄膜。用扫描电镜(SEM)、选择区电子衍射(SAED)、X射线衍射(XRD)、傅里叶红外透射谱(FTIR)和荧光光谱(PL)对样品进行形貌、结构、组分和发光特性的分析。SEM显示在平滑的表面上出现了由绳状、树根状、项链状晶体组成的奇异面。FTIR、XRD和SAED显示生成的GaN奇异面呈六方纤锌矿多晶结构同时含有少量的碳污染。PL谱显示了不同于一般GaN发光谱的发光峰。     

Quantitative analysis of small amounts of cubic GaN phase in GaN films grown on sapphire

Thin GaN films, grown by metal organic chemical vapor deposition on the basal plane of sapphire substrates, were characterized by x-ray pole figures, high-resolution x-ray diffraction and transmission electron microscopy. This combination was found sensitive to small amounts (down to 0.1%) of cubic GaN phase in specimens subjected to surface nitridation treatment prior to epitaxial growth. The presence of the cubic phase and its orientation relations to the hexagonal GaN matrix was established by means of pole figures and selected area electron diffraction. The amount of cubic phase was determined by comparing the integrated x-ray diffraction intensities of the (311) cubic GaN and the (11.2) hexagonal GaN reflections. Optimum nitridation duration was found, which corresponds to almost complete suppression of the cubic phase formation.     

GaN and AlGaN high-voltage rectifiers grown by metal-organic chemical-vapor deposition

In this paper, we report the study of the electrical characteristics of GaN and AlGaN vertical p-i-n junctions and Schottky rectifiers grown on both sapphire and SiC substrates by metal-organic chemical-vapor deposition. For GaN p-i-n rectifiers grown on SiC with a relatively thin “i” region of 2 μm, a breakdown voltage over 400 V, and forward voltage as low as 4.5 V at 100 A/cm² are exhibited for a 60-μm-diameter device. A GaN Schottky diode with a 2-μm-thick undoped layer exhibits a blocking voltage in excess of ∼230 V at a reverse-leakage current density below 1 mA/cm², and a forward-voltage drop of 3.5 V at a current density of 100 A/cm². It has been found that with the same device structure and process approach, the leakage current of a device grown on a SiC substrate is much lower than a device grown on a sapphire substrate. The use of Mg ion implantation for p-guard rings as planar-edge terminations in mesageometry GaN Schottky rectifiers has also been studied.     

Properties of GaN epitaxial layers grown at high growth rates by metalorganic chemical vapor deposition

GaN epitaxial layers were grown at high growth rates by increasing the input trimethylgallium (TMG) flow rate while keeping the NH₃ flow rate constant in metalorganic chemical vapor deposition. The electrical and optical properties of the grown layers have been investigated. With the increasing TMG flow rate, the electron concentration tends to decrease gradually and the Hall mobility decreases significantly. Considering the temperature dependence of the Hall mobility and the correlation between the Hall mobility and the electron concentration, it has been indicated that the more acceptors are incorporated and consequently the compensation ratio becomes higher with increasing the TMG flow rate. Photoluminescence measurements have revealed that the intensity ratio of the bound exciton emission to the 2.2 eV band emission, which is assumed to correlate to carbon or Ga vacancies, was decreased with increasing the TMG flow rate. It might be reasonable to take a lot of acceptor incorporation to explain the degradation of the electrical and optical properties in the samples grown at high growth rates by increasing the TMG flow rate.     

基于蓝宝石纳米图形衬底上氮化镓生长的研究

利用两步生长法在蓝宝石纳米图形衬底（NPSS）上生长得到高质量的氮化镓薄膜。通过XRD和SEM对薄膜质量的表征和研究发现,为得到高质量的氮化镓（GaN）薄膜,在NPSS上生长时得到的最优缓冲层厚度为15nm,而在微米级尺寸的图形衬底（MPSS）上得到的最优缓冲层厚度远大于15nm。同时,在NPSS上生长氮化镓薄膜的过程中观察到一个有趣的现象,即GaN在NPSS上生长的初始阶段,氮化镓晶粒主要在图形之间的平面区域生长,极少量的GaN在衬底图形的侧面上聚集生长。这一有趣的现象明显不同于GaN在MPSS上的生长过程。接着,又在NPSS上生长了GaN基LED结构,并对其光电性能进行了研究。     

Metalorganic chemical vapor deposition growth of high optical quality and high mobility GaN

Hall mobilities as high as 702 and 1230 cm²/Vs at 300 and 160K along with low dislocation densities of 4.0 × 10⁸ cm^-2 have been achieved in GaN films grown on sapphire by metalorganic chemical vapor deposition. High growth temperatures have been established to be crucial for optimal GaN film quality. Photoluminescence measurements revealed a low intensity of the deep defect band around 550 nm in films grown under optimized conditions.     

在蓝宝石图形衬底上生长低穿透性位错的GaN薄膜

使用MOCVD外延系统,采用3D-2D生长模式在圆锥图形蓝宝石衬底上生长GaN薄膜。研究发现3D-2D生长模式能够有效的减少GaN薄膜的穿透性位错,其中3D GaN层的生长条件是关键：低V/III比,低温和高生长压力。为了进一步减少TD,3D GaN层的厚度应该与图形衬底上的图形高度接近。当3D GaN层生长结束时,3D GaN层把图形衬底的图形围在其中,具有倾斜的侧壁和（0001）向的上表面,而图形上基本没有沉积物。在接下来的2D生长过程里,GaN沿倾斜侧面快速生长,使得侧面上的穿透性位错产生弯曲,从而减少GaN薄膜的穿透性位错。经过对3D条件的优化,GaN薄膜的穿透性位错降低到1×108cm-2,XRD测试得到的（002）,（102）半宽分别达到211弧秒和219弧秒。     

Structure of GaN films grown by molecular beam epitaxy on (0001) sapphire

GaN films grown by electron-cyclotron resonance plasma-assisted molecular beam epitaxy were studied by transmission electron microscopy and x-ray diffraction (XRD). Two sets of films were compared that were grown under identical conditions except for the ratio of the Ga to N flux. Films with a 30% higher Ga to N ratio (A films) were found to contain inversion domains (IDs). No IDs were found in films grown with a lower Ga to N ratio (B films), but instead the zinc-blende GaN was found near the film substrate interface. A narrower XRD rocking curve width along the (0002) direction and a broader rocking curve width along the asymmetric (1102) axis were found for A films compared to B films.     

Ten-thousand-hour operation of crank transverse-junction-stripe lasers grown by metal-organic chemical vapor deposition

For over 10 000 h, crank transverse-junction-stripe (TJS) laser diodes, grown by metal-organic chemical vapor deposition (MOCVD) maintain low and quite stable operating currents in the automatic-power-control aging with output power of 5 mW/facet at 55°C and 70°C. These MO-CVD lasers have excellent initial characteristics and small distributions of the characteristics. The reason for this excellent result is discussed in relation to the crystalline quality and the uniform growth of MO-CVD wafers.     

Initial stages of GaN/GaAs (100) growth by metalorganic chemical vapor deposition

We have investigated the growth of GaN buffers by metalorganic chemical vapor deposition (MOCVD) on GaAs (100) substrates. Atomic force microscope (AFM) and reflection high-energy electron diffraction (RHEED) were employed to study the dependence of the nucleation on the growth temperature, growth rate, annealing effect, and growth time. A two-step growth sequence must be used to optimize and control the nucleation and the subsequent growth independently. The size and distribution of islands and the thickness of buffer layers have a crucial role on the quality of GaN layers. Based on the experimental results, a model was given to interpret the formation of hexagonal-phase GaN in the cubic-phase GaN layers. Using an optimum buffer layer, the strong near-band emission of cubic GaN with full-width at half maximum (FWHM) value as small as 5.6 nm was observed at room temperature. The background carrier concentration was estimated to be in the range of 10¹³ ∼ 10¹⁴ cm⁻³.